SET_SUBAREA is not a diagnostic, but shows up as one in rose edit

[jfrost-mo]

What problem does your feature request solve?

• This is a design feature consideration.
• Strictly the subarea is not a diagnostic but a filter on the diagnostics/data so I am unsure why it is classed as a diagnostic in the GUI.
• There are multiple options to improve this depending on how the filter applies (and if there will be the option to apply multiple area filters from the same run in the future).
• It applies to all diagnostics: the subarea should be defined either as a “common” diagnostic on a “common diagnostic page” or as part of the “General” page.
• It applies to some of the diagnostics: the sub area should be defined when the diagnostic is set to true.
• It may be easier to define on general and then for each diagnostic have (when set to true) a subarea switch that uses the information if set to true on the “common”/”General” page.

Describe the solution you'd like

Subarea selection should be more clearly made into a diagnostic sub-option.

Describe alternatives you've considered

We don't want to force people to type in their subarea bounds over and over. Maybe this is a sign we need separate and reusable pre-processors like ESMValTool. But then, it is nice having everything contained within a recipe.

[jfrost-mo]

Fixed in #765.